Brain Pathology Case of the Month - December 2008

DIAGNOSIS PIGMENTED PLEOMORPHIC XANTHOASTROCYTOMA

DISCUSSION

Pleomorphic Xanthoastrocytomas (PXA) account for less than 1% of astrocytic neoplasms (10). PXAs most commonly affect those in their first and second decades, and typically present with seizures. More than 98% of PXAs are found supratentorially (1).

Imaging usually reveals the tumor mass and associated cyst. Tumors are most often superficial with meningeal attachment (10).

PXAs are a WHO grade II neoplasm with a generally favorable prognosis; survival is reported as 81% at 5 years (10). Malignant progression is, however, reported to occur in up to 20% of cases (6).

PXAs are uncommon but well known tumors. The present case was of added interest for its atypical location and for being one of only three pigmented examples reported. The two previous reported cases of pigmented PXA were suprasellar and medial temporal in location, presenting with a visual field deficit and seizures, respectively (5,7).

The present case was typical with respect to histological, immunohistochemical and ultrastructural features. Although the tumor cells expressed abundant GFAP, select cells expressed neuronal markers synaptophysin and neurofilament. The significance of this seemingly ambivalent bi-phenotypic expression pattern is unknown although some have suggested this as evidence that PXAs may be neuroglial in nature (2,3).

The feature of particular interest in the present case was the presence of melanin. Examples of pigmented primary tumors are well documented with pigments including neuromelanin, melanin and lipofuscin. Five melanotic astrocytic tumors have been previously reported. Of these five tumors; two were PXAs, one had PXA-like features, one was a ganglioglioma with PXA as the astrocytic component and one was a pilocytic astrocytoma (4,5,7,8,9). In the present case, Masson-Fontana preparations and ultrastructural examination confirmed the pigment to be melanosomal melanin. Of the five previously reported melanotic tumors, all but the pilocytic astrocytoma, were also found to contain melanosomal melanin as their pigment (5,7).

The present case is the third pigmented PXA to be reported, adding to a small collection of melanotic gliomas. While melanin production in gliomas is a curious finding, its prognostic significance is uncertain. In a twelve year follow up of one pigmented PXA there was no recurrence (5). In the present case there is no evidence of recurrence on MRI one year post-operatively.